In the activated sludge systems known in the art for the treatment of sewage, it is a common practice to provide a series of stages, each of which can be provided with an activated sludge basin followed by a separator such as a settling tank or clarifier from which sludge is separated from an effluent which can be passed to the next stage. A portion of the sludge may be recycled to the activation basin and excess sludge can be wasted or further processed. The effluent of each earlier stage can be treated in the activation basin of the next stage until a final effluent is produced in the cascade. The influent to the first stage is generally treated to remove large objects and masses, e.g. by screening, or untreated waste water can be introduced into a primary clarifier to settle out materials including high molecular weight organisms which can prove troublesome in the biological treatments which are to follow.
The activation basins can be provided with means for introducing oxygen (as air, oxygen enriched air or technically pure oxygen).
Among these biological waste-treatment systems, there are those which treat the waste water, a term which will be utilized hereinafter to mean municipal sewage, industrial waste waters and mixtures or combinations thereof unless further specified, in a two-stage activated sludge process in which the first activation stage is operated as an adsorption stage with a sludge loading of at least 2 kg BOD.sub.5 per kg of dry substance per day in an aerobic or facultative anaerobic mode and the bizonoses of the first and second activation stages are separated from one another.
The term "biozonose" is here utilized in its usual meaning to define the entire biological universe represented by the successive stages and in the sense of the term "biozone" to indicate that the biological conditions prevailing in the second or subsequent zone are not to be imported into the first or earlier zone.
A strict separation can be maintained by effective removal of sludge from the effluent of the first stage so that only the clarified effluent from the first stage enters the activation basin of the second stage and by ensuring that no sludge from the second stage clarifier is recycled to the activation basin of the first stage.
Such processes are continuous processes in the art and the adsorption stage can be denoted as an A-stage in the technical literature dealing with such treatments.
The separation of the biozonoses is effected as noted by an intermediate clarifier and by preventing sludge from the second or further activation stages from being recycled to the first high-loading activation stage.
In the adsorption stage, the sludge is maintained in a break-in phase by appropriate separation of sludge in the intermediate clarifier and its recycling to the activation basin, whereby substrate respiratory decomposition is initiated. In order to maintain the sludge in the first activation stage in this state, it is essential that the sludge age be minimized by a correspondingly controlled removal of sludge at the intermediate clarifier.
Where the volumetric loading is about 10 kg BOD.sub.5 per m.sup.3 per day in the first activation stage, it is possible to effect the splitting or cracking in adsorption and/or flocculation of the higher molecular weight compounds so that the decomposition products can be removed from the effluent and the first activation basin with the excess sludge from the first stage in the intermediate clarifier. It is thus possible to eliminate the need to utilize the energy required in other processes for the full biological degradation of these compounds.
In subsequent activation stages, the lower molecular weight and more readily biologically degradable compounds can be decomposed especially easily and rapidly.
This can be done in various ways including the so-called AB process described in German open application No. DE-OS 26 40 875 and the so-called ATB process described in German open application No. DE-OS 29 08 134. However, while these systems have been found to be generally satisfactory, it has been discovered that the first stage is particularly sensitive to various destabilizing factors such as sudden changes in pH, salt concentrations and toxic-element levels or the like.